RFE: Get list of all alerts in kapacitor

Currently there is no easy way to see all alerts (AlertNodes) in kapacitor. One can browse task by task looking for alerts, but it's really hard if task is is long.

It would be extremely nice to see something like `kapacitor list alerts` to see all alerts (with name of parental task, its state (enabled/disabled), assigned tags (`category` node) counters and current state (OK/INFO/WARN/CRITICAL)).

Currently there is no easy way to "whole picture" of monitoring in one command, and I think this command will make it possible.

Start by locating the existing kapacitor list command and the AlertNode and task entry points. Determine how to gather alerts across tasks and expose the requested task name, enabled state, category tags, counters, and OK/INFO/WARN/CRITICAL state. Done means one command provides the whole alert picture consistently.

Written by the indexing model from the issue text.